DESCRIPTION

The strerror() function returns a string describing the error code
passed in the argument errnum, possibly using the LC_MESSAGES part of
the current locale to select the appropriate language. This string
must not be modified by the application, but may be modified by a
subsequent call to perror(3) or strerror(). No library function will
modify this string.
The strerror_r() function is similar to strerror(), but is thread safe.
This function is available in two versions: an XSI-compliant version
specified in POSIX.1-2001 (available since glibc 2.3.4), and a GNU-
specific version (available since glibc 2.0). The XSI-compliant
version is provided with the feature test macros settings shown in the
SYNOPSIS; otherwise the GNU-specific version is provided. If no
feature test macros are explicitly defined, then (since glibc 2.4)
_POSIX_SOURCE is defined by default with the value 200112L, so that the
XSI-compliant version of strerror_r() is provided by default.
The XSI-compliant strerror_r() is preferred for portable applications.
It returns the error string in the user-supplied buffer buf of length
buflen.
The GNU-specific strerror_r() returns a pointer to a string containing
the error message. This may be either a pointer to a string that the
function stores in buf, or a pointer to some (immutable) static string
(in which case buf is unused). If the function stores a string in buf,
then at most buflen bytes are stored (the string may be truncated if
buflen is too small) and the string always includes a terminating null
byte.

RETURNVALUE

The strerror() and strerror_r() functions return the appropriate error
description string, or an "Unknown error nnn" message if the error
number is unknown.
The XSI-compliant strerror_r() function returns 0 on success; on error,
-1 is returned and errno is set to indicate the error.

ERRORS

EINVAL The value of errnum is not a valid error number.
ERANGE Insufficient storage was supplied to contain the error
description string.

CONFORMINGTO

strerror() is specified by POSIX.1-2001, C89, C99. strerror_r() is
specified by POSIX.1-2001.
The GNU-specific strerror_r() function is a non-standard extension.
POSIX.1-2001 permits strerror() to set errno if the call encounters an
error, but does not specify what value should be returned as the
function result in the event of an error. On some systems, strerror()
returns NULL if the error number is unknown. On other systems,
strerror() returns a string something like "Error nnn occurred" and
sets errno to EINVAL if the error number is unknown.